Aerial Oomycetes Cultural Control Review

Summary/Findings:       

The development of aerial oomycetes-associated diseases is strongly dependent upon a range of environmental conditions.

• Humidity and leaf wetness aid disease development.
  • Spore germination and infection normally requires free moisture, whilst spore production requires conditions of high relative humidity. If these conditions are not present, other environmental factors are considered to be irrelevant.
  • Overall, low humidity negatively impacts the survival of downy mildew pathogens. Attempts to reduce leaf wetness or RH (such as increased ventilation, air movement such as fanning or increased plant spacing) could therefore be effective in the reduction of infection or sporulation, respectively.
  • It is important to note, however, that a decrease in RH, if applied at the incorrect time of day, could actually trigger spore release from these pathogens.

• The amount and duration of light can affect developmental processes such as sporulation and infection in these pathogens.
  • Continuous broad- and narrow-band illumination has been shown to suppress the sporulation of many downy mildew pathogens. Continuous light application can prevent sporulation entirely in some cases, but could also decrease the latent phase of infection.
  • Night break lighting can prove effective against downy mildew diseases but is highly dependent upon the length of light and dark periods used, with the preceding dark period appearing to be of key importance.
  • Effects of white light do not pass to non-illuminated parts of the same leaf or plant. This could mean that parts of the crop which receive lower levels of illumination could prove to be problem areas. In addition, fruiting bodies may still be visible on leaves even if spores are not being produced.
  • For some pathogen species, red and blue light appear equally to be effective in control, in other species, only one of these two colours appears to be effective.
  • UV light, particularly in the UV-B (and perhaps also UV-C) ranges appear to have potential for control of these diseases, including as a ‘priming’ treatment, but careful optimisation of treatments is required to avoid crop damage.
  • Alternatively, polyethylene filters could be used to modify incoming daytime irradiance.
  • However, spore release can also be triggered by light e.g. in the morning as humidity decreases.

• The temperature of the growth environment can greatly influence the development of these diseases.
  • Temperature extremes negatively impact aerial oomycetes. Survival of downy mildew pathogens is generally considered to be negatively affected by temperatures above 35°C, but for some species of pathogen, survival can be reduced at lower temperatures.
  • Daytime solar heating, through the closing of ventilation or by covering the growth environment with polyethylene sheets to increase the temperature has been reported to be effective. The effectiveness of heat treatments appears to be dependent upon the stage of the infection.
  • Many studies regarding the effect of high temperatures on downy mildew pathogens have been performed in countries with higher climatic temperature and sunlight levels than the UK, so the ability to achieve the required temperatures necessary for pathogen inhibition in the UK could be limited. One possibility could be to employ longer duration treatments at lower temperatures or to augment with supplementary heating provision.

• An important consideration for cultural control of aerial oomycete-associated diseases is the strong degree of interaction between the different environmental factors affecting their biology.
  • Such interactions must be considered when designing effective control strategies for these diseases, which may need to combine changes in more than one environmental variable simultaneously. Maintenance of suboptimal temperatures for pathogens in growing environments could potentially allow growers to reduce disease incidence where leaf wetness duration is difficult to modify, or vice versa.
  • It must be also noted that inoculum of downy mildew pathogens spreads better when temperatures are high and RH is comparatively low. This brings clear implications for using heat or humidity to control such diseases, as whilst disease processes such as germination or infection may be reduced, spores may spread more easily, highlighting a need to carefully optimise cultural control treatments and the timing of treatment application in order to obtain maximum efficacy.

• Aerial oomycete pathogens also appear to be spread via contaminated seed.
  • Some success has been reported using hot air and hot water treatments of seed.
  • However, as only a fraction of infected seed is required for an outbreak, this approach should be combined with other control methods. Similarly, screening of incoming seed batches is unlikely to be effective in eliminating disease.

• Limited research has been carried out regarding early warning systems for vegetable disease occurrence in glasshouses. The effectiveness of such systems will likely depend on how early they are able to predict likely downy mildew outbreaks. If infection has already occurred but symptoms are not yet apparent, i.e. the disease is in its latent phase, then effectiveness may be lower and the system could be more useful for assisting in the timing of fungicide applications rather than eliminating outbreaks altogether.

Other potential cultural control approaches include:

• An increase in plant spacing, avoiding overhead irrigation, rotations, rogueing, avoiding overwatering and hygiene strategies
• Polyethylene mulches to cover the surface of growing media in order to reduce evaporation and humidity.
• Treatment of irrigation water using e.g. hydrogen peroxide or UV.
• Modification of the crop fertilisation regime.
• Maintenance of good crop hygiene, the removal of infected plants, decontamination of the growth system and if possible, the use of rotations.

Control of humidity, leaf wetness, temperature and lighting offer a number of potential strategies for combatting aerial oomycete-associated diseases. It should be stressed that the interaction between these variables remains an important consideration in the design of cultural control methodologies. Despite the promising data produced, further research into optimising novel cultural control strategies tailored to commercial setups is likely to be required.

Aims and Objectives:    

1) Collate, using a systematic methodology, existing research literature regarding the potential for culture control of aerial oomycete pathogens in protected production.

2) Critically review the existing data and identify gaps in the existing knowledge base, highlighting potential areas for future research.

3) Liaise with growers to determine the most feasible and promising courses to pursue for cultural control.

4) Generate recommendations for uses of cultural control based on existing research and grower contributions.

5) Disseminate findings of the project to the horticulture industry through a range of media.